The present invention relates to a method of measuring and controlling a gap between two objects by using a diffraction grating, a method of aligning the two objects relative to each other by utilizing the method of measuring and controlling the gap, and an apparatus for implementing these methods, these methods and apparatus being applied to an exposure apparatus for manufacturing semiconductor ICs and LSIs, a pattern evaluation apparatus, a microgap measuring apparatus, or a high precision aligner.
An X-ray exposure apparatus has been developed as an apparatus for producing submicron patterns in a mass production line and for micropatterning of semiconductor ICs and LSIs. In a conventional X-ray exposure apparatus using a divergent X-ray source, high-precision alignment (to be referred to as an alignment or transverse alignment hereinafter) of a predetermined position of a mask with a corresponding position of a wafer must be inevitably performed in a two-dimensional manner. At the same time, another alignment must be performed wherein a distance, i.e., a gap between the mask and the wafer is set at a predetermined value with high precision.
A conventional transverse alignment method is described as a dual diffraction grating method in J. Vac. Sci. Technol. Vol. 19, No. 4, NOV/DEC 1981, pp. 1214-1218. According to such a dual diffraction grating method, a laser beam from a laser source is incident on a positional error detection mask mark formed on the mask, reflected by a positional error detecting wafer mark formed on the wafer, and then again passes through the positional error detection mask mark. The mask and wafer marks comprise diffraction gratings, respectively. More specifically, the wafer mark comprises a transmission diffraction grating, while the wafer mark comprises a reflection diffraction grating.
Among the light components diffracted by the positional error detecting mask and wafer marks, positive and negative first-order diffracted light components which are diffracted symmetrically about the incident light are incident on photoelectric transducers, respectively. The photoelectric transducers convert the reflected light intensity components I+1 and I-1 to electrical signals. A difference .DELTA.I(=I+1-I-1) between these signals is calculated, and transverse alignment can be performed in accordance with the difference .DELTA.I. The difference .DELTA.I comprises a repeating waveform in synchronism with a pitch P0 of the diffraction grating. When the two diffraction gratings completely match with each other (i.e., a relative positional error d=0) or the relative positional error d therebetween is P0/2, the difference .DELTA.I is zero irrespective of the gap between the mask and the wafer. Therefore, the stage is moved to perform an alignment such that the difference .DELTA.I is set to zero.
A gap is set such that the gap is measured by a capacitive gap sensor arranged around the mask. However, a curve representing changes in the difference .DELTA.I with respect to the relative positional error d greatly changes even if the gap Z is slightly changed. For example, according to the above-mentioned reference, when a change in the difference .DELTA.I with respect to the relative positional error d at a laser beam wavelength .lambda.=0.6328 .mu.m and the pitch P0=1.1 .mu.m is obtained, a curve for Z=20.02 .mu.m becomes a smoothly period curve, however it for Z=20.05 .mu.m includes many upper and lower peaks and many zero-crossing points. For this reason, alignment control requires a long time, thus hindering high precision alignment. Theoretically, high precision alignment is performed by using a .DELTA.I curve for Z=20.02 .mu.m. For this purpose, the gap must be highly accurate, and variations must be minimized. However, no conventional apparatus satisfies such a need. Demand has thus arisen for a gap measuring/control method of precisely measuring and controlling the gap and a gap measuring/control apparatus for implementing such a method.
Since the above-mentioned gap sensor has a large size, it is difficult to measure the gap in the vicinity of the diffraction gratings for measuring the positional error. When the wafer or the mask has poor flatness, gap measurement is performed in the vicinity of the peripheral portion of the mask. Even if the gap immediately under the gap sensor is accurately measured by the above-mentioned gap sensor, the gap between the positional error detecting mask and wafer marks cannot be always set at an optimal value, resulting in inconvenience. Therefore, it is difficult to perform high precision alignment control in accordance with the dual diffraction grating method. Therefore, demand has thus arisen for a gap measuring/control method for measuring and controlling with high precision the gap in the vicinity of the positional error detecting marks and a gap measuring/control apparatus for implementing such a method.